An ink jet printer or another conventional printer includes a sheet feeder, a sheet conveyer, a print unit and a sheet discharger. The feeder includes a sheet support. The print unit includes a print head. The feeder can feed one after one the sheets of paper stacked on the support. The conveyer can convey the fed sheet toward the print head. The print unit can print the conveyed sheet. The discharger can discharge the printed sheet from the printer. The feeder also includes feed rollers for rotation in frictional contact with the top one of the sheets stacked on the support. The feeder further includes a drive for rotating these feed rollers. The conveyer includes a feed roller which can be rotated by the drive. The conveyer also includes a pressure roller in rotatable compressive contact with the conveyer feed roller.
In order for the top one of the sheets stacked on the support to be fed by the feeder and conveyed by the conveyer toward the print head, the drive first rotates the feeder feed rollers in the normal direction to feed the top sheet toward the conveyer. At the same time, the drive rotates the conveyer feed roller reversely to stop the front end of the fed sheet temporarily between the conveyer feed roller and the pressure rollers. Then, the conveyer feed roller is rotated normally, with the feeder feed rollers rotated normally, to convey the sheet toward the print head.
When the feeder feeds a sheet of paper toward the conveyer, the conveyer feed roller is rotated reversely to stop the front end of the sheet temporarily, as stated above, in order to warp or curl the sheet between the feeder and the conveyer. With the sheet warped, the position and direction of its front end are adjusted securely with respect to the conveyer feed roller. Thereafter, in accordance with the amount of normal rotation of the conveyer feed roller, the feed from this roller toward the print head is controlled precisely. Therefore, in this type of drive, the feeder feed rollers can be rotated normally and the conveyer feed roller can be rotated reversely at the same time. Besides, the feeder feed rollers and conveyer feed roller can be rotated normally at the same time.
FIGS. 14 through 16 show the drive for an ink jet printer which is disclosed by the assignee of the present application in U.S. patent application Ser. No. 08/769,277. The printer includes semi-cylindrical feed rollers 101 and another feed roller 104. The drive 100 includes a feed motor 107, which can rotate a feed gear 105 normally and reversely through a reduction gear 106. The feed gear 105 is fixed to one end of the shaft to which the feed roller 104 is fixed. The feed gear 105 is in mesh with a sun gear 110, which is fixed to a pin. A first carrier arm 111 and a second carrier arm 115 are supported pivotably on the pin. A first planet gear 112 is supported rotatably on the free end of the first arm 111, and in mesh with the sun gear 110. The first planet gear 112 can mesh with an intermediate gear 113, which is in mesh with a first gear 102. A second planet gear 116 is supported rotatably on the free end of the second arm 115, and in mesh with the sun gear 110. The second planet gear 116 is urged toward a second gear 103 by a spring 115a, and can mesh with the second gear 103. The first gear 102 and second gear 103 are fixed to one end of the shaft to which the feed rollers 101 are fixed. The second gear 103 is a partial gear having a cylindrical or peripheral surface 103a without teeth. The diameter of the surface 103a is smaller than the outer diameter of the second gear 103. In order to feed a sheet of paper, a controller makes the motor 107 rotate reversely by a first predetermined amount and, thereafter, normally by a second predetermined amount.
When the feed rollers 101 are at their initial position shown in FIG. 14, reverse rotation of the feed motor 107 in the direction "a" rotates the feed gear 105 reversely in the direction "c" together with the feed roller 104. This moves the first planet gear 112 into engagement with the intermediate gear 113. As a result, the first gear 102 rotates together with the feed rollers 101 normally in the direction indicated in FIG. 14. The reverse rotation of the motor 107 by the first predetermined amount feeds toward the sheet conveyer the top one of the sheets stacked on the sheet support. While the motor 107 is rotating reversely by the first amount, the second planet gear 116 faces the peripheral surface 103a of the second gear 103. At the end of the reverse rotation by the first amount, the leading tooth of the second gear 103 comes in contact with the second planet gear 116, but the second gear 103 does not mesh with this planet gear 116 because these gears are rotating in the same direction.
Subsequently, normal rotation of the feed motor 107 in the direction "b" rotates the feed gear 105 normally in the direction "d" together with the feed roller 104. This moves the first planet gear 112 from the intermediate gear 113. At the same time, the second planet gear 116 meshes with the second gear 103. This rotates, through the sun gear 110 and second planet gear 116, the second gear 103 together with the feed rollers 101 normally in the indicated direction. When the motor 107 has rotated normally by the second predetermined amount, the peripheral surface 103a of the second gear 103 faces the second planet gear 116. As a result, this planet gear 116 disengages from the teeth of the second gear 103. Consequently, the feed rollers 101 stop at their initial position, while only the feed roller 104 is feeding the sheet.
By the way, the semi-cylindrical feed rollers 101 may be away from their initial position by having been turned by one's finger or sheets of paper touching one or more of these rollers when the sheets were supplied to the feeder, or by other cause. In this case, even when the feed motor 107 has rotated by the first amount as stated above, it is impossible to feed the top sheet by the amount corresponding to the amount of turning of the feed rollers 101. Irrespective of the initial position of the feed rollers 101, the motor 107 rotates reversely by the first amount and normally by the second amount continuously. Therefore, sometimes, it is impossible to return the feed rollers 101 to their initial position. In this case, not only the first, but also the second sheet is fed defectively. It is, however, difficult to confirm from the outside if the feed rollers 101 are out of their initial position. It is also difficult to return the rollers 101 manually with precision to the initial position.